Soil Physics

Abstract

Soil physics deals with physical properties of soils such as soil texture, porosity, soil water, soil aeration, soil temperature, soil structure, and the influence of these properties on plant growth. Soil texture refers to the particle-size distribution of soils. The primary soil particles are arbitrarily divided into different size classes. The International Society of Soil Science defines soil particles larger than 0.02 mm and smaller than 2 mm as sand, those larger than 0.002 mm but smaller than 0.02 mm as silt, and those smaller than 0.002 mm as clay. Soil particles larger than 2 mm, such as gravel and stones, are called coarse fragments and are not part of the soil itself, to which the term soil texture applies, but can have considerable influence on soil properties and plant growth. Sand particles (0.02-2 mm) can be further divided into fine sand (0.02-0.2 mm) and coarse sand (0.2-2 mm). Sand particles can be rounded or angular, and are noncohesive. They usually consist of a single mineral, usually quartz (SiO2) or other primary silicate, and may appear brown, yellow, or red as a result of Fe-oxide coatings. Due to its mineral composition, sand has a smaller plant-nutrient content than finer soil particles. Sand particles have large voids between them which promote drainage of water and entry of air into the soil. Due to their low specific surface area, sand particles can hold little water, therefore rain needs to be received at short intervals to enable plant growth on sandy soils. Silt particles (0.002-0.02 mm) do not feel gritty when rubbed between fingers and are not visible to the unaided eye as sand particles are. Quartz is generally the dominant mineral. However, when silt is composed of weatherable minerals, the release of plant nutrients can be significant. The pores between silt particles are smaller and more numerous than those in sand, and silt therefore retains more water than sand, which helps to sustain plant growth. Silt itself does not exhibit much stickiness or plasticity and is therefore easily washed away by water. If silt fractions have some cohesion and adsorptive capacity, it is due to a film of adhering clay particles.